A closed cell, particularly a cylindrical Ni-Cd storage cell has been constructed generally in the form shown in FIG. 1. In the figure, a cap type positive terminal 1 and a dish type opening seal member 2 are formed of stainless steel sheet or nickel-plated steel sheet, and placed in a valve chamber 3 formed between the two is an elastic valve body 4 like a go stone having upper and lower symmetrical surfaces similar to a convex lens as shown in FIGS. 2A and 2B, said valve body being made of an alkali-resisting rubber, e.g., chloroprene rubber, ethylene propylene rubber, fluororubber or silicone rubber, the peripheral edge of said dish type opening seal member 2 being crimped to fix the cap type positive terminal 1 in position, thereby forming a safety valve device. When the pressure of the gas in a within-cell space 8 defined by a metal cell container 6 storing a cell element 5 and serving as the negative terminal of the cell, the safety valve device serving as the positive terminal of the cell and a resin ring 7 for isolating the positive and negative poles and hermetically sealing the container rises above a predetermined value, the gas pushes up the elastic valve body 4 previously closing a gas discharge hole 9 in the dish type opening seal member 2, and discharges outside passing through the valve chamber 3 and then through a gas discharge hole 9' in the cap type positive terminal 1. If the pressure in the cell container is thereby decreased below the predetermined value, the elastic valve body 4 closes the gas discharge hole 9 of the dish type opening seal member 2 again and keeps the closed cell sealed. The gas pressure in the within-cell space 8 which exists when the gas in the within-cell space has been discharged outside is called the safety valve working pressure, which must be lower than the deforming pressure for the cell container 6 but higher than the gas pressure which rises during normal use of the cell.
Usually, the upper limit of the safety valve is 30 kg/cm.sup.2 and the lower limit 10 kg/cm.sup.2, the pressure range being about 20 kg/cm.sup.2, and it is necessary to set the safety valve working pressure in this range with less variation.
In the safety valve device described above, the valve chamber 3 is substantially filled with the elastic valve body 4, as shown in FIG. 1. FIG. 3 shows a characteristic obtained by changing the outer diameter a of the valve body shown in FIG. 2 to change the percentage of the volume occupied by the valve body in the valve chamber while making the height of the safety valve device substantially constant. As shown in this figure, it is seen that in the case where the percentage of the volume occupied by the elastic valve body in the valve chamber volume (hereinafter referred to as the valve body occupancy percentage) is high as usual, the safety valve working pressure changes greatly with a slight change in the volume of the elastic valve body, e.g., in the height of the elastic valve body.
The valve body occupancy percentage could be decreased by making the diameter a of the elastic valve body smaller than the valve body receiving section inner diameter b (hereinafter referred to as the valve body inner diameter) of the cap type positive terminal 1. However, simply making the diameter a smaller than the valve body receiving section inner diameter b would result in the elastic valve body being deviated from the center of the valve chamber; in this case, it sometimes happens that the cap type positive terminal is fixed without completely closing the gas discharge hole 9 of the dish type opening seal member 2. The safety valve working pressure in this case becomes lower than when the elastic valve body is positioned in the center of the valve chamber, forming a cause of an increased variation in the safety valve working pressure. FIG. 4 shows the relation between the ratio of the outer diameter a of the elastic valve body to the valve chamber inner diameter b, or the ratio a/b (in percentage), and the safety valve working pressure.
As is clear from the above description, with the conventional safety valve device it has been impossible to decrease the variation in the safety valve working pressure while decreasing the valve body occupancy percentage. SUMMARY OF THE INVENTION
An object of the invention is to provide a safety valve device which elimninates the aforesaid drawback, decreases the valve body occupancy percentage, and minimizes the variation in the safety valve working pressure.
The safety valve device of the invention comprises a valve chamber defined between a cap type positive terminal and a dish type opening seal member, and an elastic valve body having a polygonal upper surface with not more than eight angles, said elastic valve body being placed in said valve chamber so that the ratio (a/b) of the circumscribed circle diameter a of the valve body to the valve chamber inner diameter b is 90-105 percent. Preferably, the upper and lower surfaces of the valve body are curved and the angles are rounded. It is also preferable that the location where the gas discharge hole of the dish type opening seal member contacts the valve body is coated with a water-repelling non-drying sealing agent to prevent leakage of liquid. Further, the cap type positive terminal is fixed in position by crimping the peripheral edge of the dish type opening seal plate, and the safety valve working pressure is 10-30 kg/cm.sup.2 with less variation in this range.